Polymer solar cells promise a variety of new applications for PV, such as integration in clothing, product packages (Figure 1), curtains, or wall paper. For this to happen, the flexible, light-weight solar cells should be printed using low-cost, large-scale processes. Current industrial silicon solar cells achieve efficiencies of 15-18%, whereas organic solar cells (OSC) have to cope with 4-8% power-conversion efficiencies. Yet, industry is very interested in organic solar cells because of their mechanical flexibility and large-scale, low-cost production potential. Moreover, they operate well under low illumination conditions and non-perpendicular light angles. These are enough reasons to start dreaming about PV-integrated product packages, windows, backpacks, etc.
Low cost and large scale

Organic solar cells come in many flavors. The two largest groups are the small-molecule OSC and the polymer-based OSC. The small-molecule OSCs are typically evaporated with a planar structure, similar to the junctions in silicon solar cells. Polymer–based OSCs, on the other hand, are processed from a solution in a different structure—a bulk heterojunction structure in which the donor and acceptor material are fully intermixed. This article deals with the polymer-based OSC.

The low-cost potential of polymer solar cells is partly based on the amount of material used for the active layer. Although the polymers are rather expensive, the active layer is very thin. It is 0.1-0.3 µm as compared to the 100- to 200-µm active layer in silicon solar cells. This explains the extensive difference in cost per square meter between silicon and polymer solar cells.

Large-scale production potential contributes to the cost-efficiency of polymer solar cells. Today, researchers are studying the usability of different deposition techniques. In general, the polymers are dissolved in a solvent and deposited onto a low-cost substrate (glass or plastic). When the solvent evaporates, a very thin active layer remains with the bulk heterojunction structure.

Deposition techniques for polymer solar cellsSpin coating uses a solution on a substrate rotating at high speed to spread the fluid uniformly. It is the most commonly used technique because it produces very flat films with reproducible thickness. It is a valuable technique for the R&D on polymer solar cells to characterize and optimize polymer mixtures, solvents, and deposition parameters. The disadvantage of spin coating is that it’s limited to small areas and is not scalable to roll-to-roll production, which is a vital step towards industrial mass production of polymer solar cells.